The present invention relates to a family of catalysts and to a process for the synthesis of saturated carboxylic esters from alkyl formate and an olefin.
It is known that saturated carboxylic esters can be prepared by the reaction of alkyl formate and an olefin under pressure, at relatively high temperature and in the presence of a catalytic system comprising a transition metal derivative. For example, Patent EP-A-106,656 describes the reaction of methyl formate with ethylene in acetic acid as a solvent, at 200.degree. C. and under a pressure of 25 bars, in the presence of a catalytic system comprising iridium trichloride, methyl iodide as a promoter and para-toluenesulphonic acid as a copromoter; after 30 minutes' reaction, the reaction mixture contains, on the one hand 10.5% of unconverted methyl formate, and on the other a mixture of products containing 12.7% of methyl propionate, thereby enabling the activity of the catalyst with respect to methyl propionate, defined as the number of moles of methyl formate converted to methyl propionate per mole of catalyst per hour, to be calculated as approximately 330 h.sup.-1. In this process, not only is the selectivity for methyl propionate very low, but also the scarcity and cost of iridium make the synthesis of methyl propionate uneconomical. The publication by W. KEIM and J. BECKER in Journal of Molecular Catalysis, 54 (1989) 95-101 describes the same reaction performed for 20 hours at 230.degree. C. at 90 bars in toluene in the presence of Ru.sub.3 (CO).sub.12 as a catalyst: under these conditions, the conversion is 100% and the selectivity for methyl propionate 92%, thereby enabling the activity of the catalyst with respect to methyl propionate, as defined above, to be calculated as 230 h.sup.-1. This technique represents an advance with respect to that of Patent EP-A-106,656, inasmuch as an excellent selectivity may be obtained by means of a ruthenium catalyst which is more readily available commercially than that of iridium. However, the activity of the catalyst with respect to methyl propionate remains moderate, and even less than that of the patent already cited.
HIDAI et al. have published, in Journal of Molecular Catalysis, 40 (1987) 243-254, the reaction, performed in the presence of various catalytic systems based on ruthenium and iodides, of ethylene with an equimolar mixture of carbon monoxide and methanol. The publication shows that, at 190.degree. C. and in the presence of Ru.sub.3 (CO).sub.12 as a catalyst, the best results are obtained using sodium iodide or lithium iodide as a promoter in preference to phenyl iodide, tetra-n-butylammonium iodide or tetraphenylphosphonium iodide. The selectivity is good and the catalytic activity with respect to methyl propionate increases with the mole ratio of sodium iodide to ruthenium, being capable of reaching 1220 h.sup.-1 when the I/Ru ratio is equal to 10. This process has, however, two serious drawbacks: on the one hand the reaction necessitates recourse to gaseous carbon monoxide, the handling and transport of which are dangerous, thereby having the effect of limiting the production of methyl propionate to locations in proximity to industrial sites producing this gas; and on the other hand the use of large quantities of iodides is a well-known cause of corrosion of the steel reactors in which the reaction would be carried out.